A teenage boy who was brutally attacked before being drowned in a bathtub was allegedly killed because he wet his pants.

Kristy Bamu, 15, was allegedly accused of witchcraft by his sister Magalie and brother-in-law Erik Bikubi, according to British media reports.

The boy, who sustained 101 injuries before he drowned in a bathtub, was trying to hide his soiled pants in the kitchen when he was attacked by his siblings, the Old Bailey heard.

Another sister, Kelly Bamu, reportedly broke down in court as she accused her sister of torturing the boy.

When asked what had sparked the incident, Kelly told the jury: "The reason was Kristy had wet his pants. He tried to hide his pants in the kitchen and that is what triggered everything.

"All over a pair of pants."

It was alleged that the boy was tortured for four days after and was drowned in a bath at Magalie's flat in Forest Gate, East London on Christmas Day.

Kristy, two of his brothers and two of his sisters were all brutally attacked during the four day period due to the accusations of witchcraft and black sorcery, the court was told.

Kelly and her brothers and sister had gone to London from their home near Paris to celebrate Christmas with Magalie.

Kelly said that there had been a "nice atmosphere" in the days leading up to the brutal attack, but that changed when she heard noises coming from the kitchen.

"I didn't see anything but I heard some noise - it was from morning to night time," she said.

"There was a noise like there were plates falling."

When she faced her sister and brother-in-law, she found them making accusations of witchcraft, it was alleged.

The attack was linked to the Congolese belief that witchcraft, or Kindoki, is a negative or malicious force, the court heard.

"They started talking about kindoki, witchcraft and this and that. It was as if they were obsessed by witchcraft and then it became absolutely unbearable.

"They asked if we were witches. I repeated again and again and again that we were not witches. I did not know what was going on in their minds. They decided we had come there to kill them."

The court heard the five were forced to hold a vigil, fast and pray over the next few days to remove the kindoki, but Bikubi then allegedly said he would use a stick to get the truth out of them.

"He began to hit Kristy while my sister was watching and didn't do anything," Kelly said.

"I begged him, we didn't do anything, we are innocent. She didn't argue at all. It was as if it was completely normal. She was just sitting there as a spectator.

"He hit and hit Kristy. He was not feeling well, he was having trouble breathing and he fell over. As far as Eric and Magalie were concerned, that was the kindoki coming out of him."